1. Field of the Invention
The present invention relates to a two-layer flexible substrate, and more particularly, to a two-layer flexible substrate in which a copper layer is formed on an insulator film.
2. Description of the Related Art
Two-layer flexible substrates have attracted attention as substrates used in the manufacture of flexible printing wiring boards. Two-layer flexible substrates, in which a copper conductor layer is provided directly on an insulator film without using an adhesive, are advantageous in that the substrate can be made thinner thereby, while allowing the arbitrary adjusting of the thickness of the copper conductor layer to be formed. Such two-layer flexible substrates are ordinarily manufactured by forming an underlying metal layer on an insulator film, followed by electrolytic copper plating. However, underlying metal layers obtained that way exhibit numerous pinholes and areas where the insulator film is exposed. In the case where a thin-film copper conductor layer is provided thereon, the thin-film copper conductor layer fails to bury the exposed areas of the pinholes, with pinholes occurring also on the surface of the copper conductor layer, all of which may give rise to wiring defects. To solve the above problem, there have been proposed methods for forming a two-layer flexible substrate, for instance as disclosed in Japanese Patent Publication No. H10-193505A, comprising the steps of forming an underlying metal layer on an insulator film by dry plating, forming next a primary electrolytic copper plating layer on the underlying metal layer, carrying out then an alkaline solution treatment, forming thereafter an electroless copper plating layer, and forming lastly a secondary electrolytic copper plating layer. Such a method, however, relies on a complex set of operations.
There is a growing requirement for copper layers to enable fine patterning, on account of the trend in recent years towards printed wiring boards having higher densities, narrower circuit widths, and higher layer counts. Fine patterning thus requires copper layers having high etching rates and uniform solubility, i.e. copper layers having excellent etching characteristics.
Upon formation of wiring through coating of a resist on the copper layer and further plating, the high gloss of the copper surface may give rise to resist delamination. Two-layer flexible substrates must thus have an excellent adherence to the resist.